Stable catalysts for electroless metallization

ABSTRACT

Aqueous catalysts of nanoparticles of precious metals and stabilizers of flavonoid derivatives are used to electrolessly plate metal on non-conductive substrates. Such substrates include printed circuit boards.

FIELD OF THE INVENTION

The present invention is directed to stable aqueous precious metalcatalysts for electroless metallization. More specifically, the presentinvention is directed to stable aqueous precious metal catalysts forelectroless metallization which are tin free and are stabilized byflavonoid derivatives.

BACKGROUND OF THE INVENTION

Electroless metal deposition is a well-known process for depositingmetallic layers on substrate surfaces. Electroless plating of adielectric surface requires the prior application of a catalyst. Themost commonly used method of catalyzing or activating dielectrics, suchas non-conductive sections of laminated substrates used in themanufacture of printed circuit boards, is to treat the substrate with anaqueous tin/palladium colloid in an acidic chloride medium. Thestructure of the colloid has been extensively studied. In general, thecolloid includes a palladium metal core surrounded by a stabilizinglayer of tin(II) ions, essentially a shell of SnCl₃ ⁻ complexes whichact as surface stabilizing groups to avoid agglomeration of the colloidsin suspension.

In the activation process the tin/palladium colloid catalyst is adsorbedonto a dielectric substrate, such as epoxy or polyimide containingsubstrate, to activate electroless metal deposition. Theoretically thecatalyst functions as a carrier in the path of electron transfer fromreducing agents to metal ions in the electroless metal plating bath.Although performance of electroless plating is influenced by manyfactors, such as additive composition of the plating solution, theactivation step is key for controlling the rate and mechanism ofelectroless plating.

In recent years, along with the reduction in size and desired increasein the performance of electronic devices, the demand for defect freeelectronic circuits in the electronic packaging industry has becomehigher. Although the tin/palladium colloid has been commercially used asan activator for electroless metal plating for decades and has givenacceptable service, it has many disadvantages which are becoming morepronounced as the demand for higher quality electronic devicesincreases. The stability of the tin/palladium colloid is a majorconcern. As mentioned above the tin/palladium colloid is stabilized by alayer of tin(II) ions and its counter anions can prevent palladium fromagglomerating. The catalyst is sensitive to air and readily oxidizes totin(IV), thus the colloid cannot maintain its colloidal structure. Thisoxidation is further promoted by increase in temperature and agitationduring electroless plating. If the concentration of tin(II) falls tocritical levels, such as close to zero, palladium metal particles growin size, agglomerate and precipitate, thus becoming catalyticallyinactive. As a result there is an increase in demand for a more stablecatalyst.

Considerable efforts have been made to find new and improved catalysts.Because of the high cost of palladium, much effort has been directedtoward development of palladium free catalysts, such as colloidal silvercatalysts. Another direction that research has taken is towards a tinfree palladium catalyst since stannous chloride is costly and theoxidized tin requires a separate acceleration step. The accelerationstep is an extra step in the metallization process and it often stripsoff some catalyst on substrates, especially on substrates of glassfiber, causing voids on the plated substrate surface. However, such tinfree catalysts have shown to be insufficiently active and reliable forthrough-hole plating in printed circuit board manufacture. Further, suchcatalysts typically become progressively less active upon storage, thusrendering such catalyst unreliable and impractical for commercial use.

Alternative stabilizing moieties for tin complexes, such aspolyvinylpyrrolidone (PVP) and dendrimers, have been investigated.Stable and uniform PVP protected nanoparticles have been reported byvarious research groups in the literature. Other metal colloids, such assilver/palladium and copper/palladium in which palladium is partiallyreplaced by less expensive metals have also been reported in theliterature; however, such alternative catalysts have not beencommercially acceptable. Ionic palladium variants have been usedcommercially, but they require an extra reducing step. Accordingly,there is still a need for a stable and reliable electroless metalplating catalyst.

SUMMARY OF THE INVENTION

Aqueous catalyst solutions include one or more reducing agents andnanoparticles of one or more precious metals and one or more flavonoidglycosides and hydrates thereof.

Methods include providing a substrate; applying an aqueous catalystsolution to the substrate, the aqueous catalyst solution includes one ormore reducing agents and nanoparticles of one or more precious metalsand one or more flavonoid glycosides and hydrates thereof; andelectrolessly depositing metal onto the substrate using an electrolessmetal plating bath.

The catalysts may be used to electrolessly plate metals on substrates,including substrates of dielectric materials and are stable upon storageas well as during electroless metal plating since they do not readilyoxidize as compared to conventional tin/palladium catalysts. Theflavonoid glycoside stabilizers function as do stannous chloride inconventional tin/palladium catalysts except that the flavonoid glycosidestabilizers are biodegradable, thus they do not present an environmentalhazard as does stannous chloride upon disposal. The raw materials usedto make the stabilizers are readily available from plant life which isessentially ubiquitous. The flavonoid glycoside stabilized preciousmetal catalysts enable electroless metal plating without an accelerationstep, reduce or eliminate interconnect defects and enable good metalcoverage of the substrate, even walls of through-holes of printedcircuit boards.

DETAILED DESCRIPTION OF THE INVENTION

As used throughout this specification, the abbreviations given belowhave the following meanings, unless the context clearly indicatesotherwise: g=gram; mg=milligram; ml=milliliter; L=liter; cm=centimeter;m=meter; mm=millimeter; μm=micron; nm=nanometers; ppm=parts per million;° C.=degrees Centigrade; g/L=grams per liter; DI=deionized;ICD=interconnect defects; wt %=percent by weight; and T_(g)=glasstransition temperature.

The terms “printed circuit board” and “printed wiring board” are usedinterchangeably throughout this specification. The terms “plating” and“deposition” are used interchangeably throughout this specification. Theterms “a” and “an” refer to the singular and the plural. All amounts arepercent by weight, unless otherwise noted. All numerical ranges areinclusive and combinable in any order except where it is logical thatsuch numerical ranges are constrained to add up to 100%.

Aqueous nanoparticle colloidal catalyst solutions include one or morereducing agents, and nanoparticles of one or more precious metals andone or more flavonoid glycosides or hydrates thereof. Flavonoidglycosides or hydrates thereof stabilize the precious metal catalyst.Flavonoid glycosides are compounds which include a flavonoid typecompound joined to one or more carbohydrates by a glycosidic bond.Flavonoids include but are not limited to flavanones, flavanonols,flavanonals, flavonols, flavanols, isoflavonoids and neoflavonoids.Carbohydrates include, but are not limited to monosaccharides such ashexoses including aldohexoses and ketohexoses and oligosaccharides suchas disaccharides. Flavonoid glycosides of the precious metal catalystsmay have a general formula:

where R₁, R₂, R₃ and R₄ are independently hydrogen, hydroxyl, linear orbranched (C₁-C₆)alkoxy, linear or branched (C₁-C₆)acetate or—O-carbohydrate, preferably one of R₁, R₂, R₃ and R₄ is —O-carbohydrate;the dotted lines of ring B are optional double bonds; Z₁ is >C═O or astructure having formula:

where R₅ and R₆ are independently hydrogen, or ring C, when Z₁ forms adouble bond with the carbon at position 3 of ring B, only one of R₅ andR₆ is present and joined to the carbon at position 2, preferably R₅ orR₆ is ring C; Z₂ is a structure having formula:

where R₇ and R₈ are independently hydrogen, hydroxyl, ring C or—O-carbohydrate, when Z₂ forms a double bond with the carbon at position2 or 4 of ring B, only one of R₇ and R₈ is present and joined with thecarbon of position 3; Z₃ is >C═O or a structure having formula:

where R₉ and R₁₀ are hydrogen or ring C, when Z₃ forms a double bondwith the carbon at position 3 of ring B, only one of R₉ and R₁₀ ispresent and joined to the carbon at position 4, preferably Z₃ is >C═O;and

R_(1′), R_(2′), R_(3′), R_(4′) and R_(5′) of ring C are independentlyhydrogen, hydroxyl, linear or branched (C₁-C₆)alkoxy, linear or branched(C₁-C₆)acetate or —O-carbohydrate, preferably at least one ofR_(1′)-R_(5′) is hydroxyl. At least one of ring A, B or C includes—O-carbohydrate, preferably ring A or B is bonded to —O-carbohydrate,and there is at least one hydroxyl on ring A or ring C, preferably ringA and ring C include at least one hydroxyl group.

Preferred flavonoid glycosides include compounds where at least one ofR₁-R₄ of ring A is hydroxyl and ring A may include a —O-carbohydrate;ring B may or may not include a double bond, when it does include adouble bond, preferably, the double bond is between the carbons atposition 2 and position 3; Z₁ may include hydrogen or ring C, Z₂ mayinclude hydrogen or a —O-carbohydrate, preferably —O-carbohydrate isjoined to ring B at Z₂, position 3; Z₃ is >C═O and ring C includes atleast one hydroxyl group and may further include (C₁-C₃)alkoxy andhydrogen. Examples of such compounds are Rutin, robinin, diosmin andhydrates thereof.

More preferred flavonoid glycosides include compounds where at least oneof R₁-R₄ is hydroxyl and a —O-carbohydrate, preferably, only one ofR₁-R₄ is hydroxyl and —O-carbohydrate with the remainder hydrogen; ringB does not include any double bonds or hydroxyl groups, Z₁ is formula(II) and Z₂ is formula (III) where at least one of R₇-R₁₀ is ring C,preferably, R₇ or R₈ is ring C with the remainder hydrogen and Z₃is >C═O; at least one of R_(1′)-R_(5′) of ring C is hydroxyl,preferably, only one of R_(1′)-R_(5′) is hydroxyl with the remainderhydrogen or hydrogen and at least one (C₁-C₃)alkoxy. Examples of suchflavonoid glycosides are flavanone glycosides such as naringin,hesperidine and hydrates thereof.

Carbohydrates include but are not limited to hexoses such as aldohexosessuch as allose, altrose, galactose, glucose, gulose, iodose, mannose,rhamnose and talose, ketohexoses such as fructose, psicose, sorbose andtagatose; pentoses such as ribose, ribulose and deoxyribose;oligosaccharides such as disaccharides such as saccharose, lactose,maltose, rutinose and trehalose. Preferably the carbohydrates arehexoses such as galactose, glucose and rhamnose and the disacchariderutinose, more preferably the carbohydrates are glucose, rhamnose andrutinose.

The flavonoid glycosides are included in the aqueous catalyst insufficient amounts to provide particle stabilization. Mixtures of thevarious stabilizing flavonoid glycosides described above may be includedin the aqueous catalysts. Minor experimentation may be done to determinethe amount of a particular stabilizer or combination of stabilizers tostabilize a catalyst. In general, one or more stabilizing compounds areincluded in the aqueous catalyst in amounts of 10 mg/L to 5 g/L,preferably from 0.2 g/L to 2 g/L.

One or more reducing agents are included to reduce metal ions to metal.Conventional reducing agents known to reduce metal ions to metal may beused. Such reducing agents include, but are not limited to dimethylamineborane, sodium borohydride, ascorbic acid, iso-ascorbic acid, sodiumhypophosphite, hydrazine hydrate, formic acid and formaldehyde. Reducingagents are included in amounts to reduce substantially all of the metalions to metal. Such amounts are generally conventional amounts and arewell known by those of skill in the art. Typically reducing agents areincluded in the aqueous catalyst solution in amounts of 50 mg/L to 500mg/L.

Sources of precious metals include any of the conventional water solubleprecious metal salts known in the art and literature which provideprecious metals having catalytic activity. Mixtures of two or morecatalytic precious metals may be used. Such salts are included toprovide metal in amounts of 10 ppm to 2000 ppm, preferably from 20 ppmto 500 ppm. Palladium salts include, but are not limited to palladiumchloride, palladium sodium chloride and palladium potassium chloride.Silver salts include, but are not limited to silver nitrate, silverfluoride, silver oxide, silver p-toluenesulfonate, silver sodiumthiosulfate and silver potassium cyanide. Gold salts include, but arenot limited to gold cyanide, gold trichloride, gold tribromide,potassium gold chloride, potassium gold cyanide, sodium gold chlorideand sodium gold cyanide. Platinum salts include, but are not limited toplatinum chloride and platinum sulfate. Iridium salts include, but arenot limited to, iridium tribromide and iridium potassium chloride.Typically the salts are palladium, silver, gold and platinum. Preferablythe salts are silver, palladium and platinum. More preferably the saltsare palladium and silver.

Optionally, one or more antioxidants may be included in the aqueouscatalyst solutions. Conventional antioxidants may be included and may beincluded in conventional amounts. Typically antioxidants are included inamounts of 0.1 g/1 to 10 g/l, preferably from 0.2 g/L to 5 g/L. Suchantioxidants include, but are not limited to, ascorbic acid, phenolicacid, polyphenolic compounds, such as but not limited to, hydroxybenzoicacid and derivatives, gallic acid, hydroxybenzoaldehydes, catechol,hydroquinone and catechin.

The components which make up the aqueous catalyst may be combined in anyorder. Any suitable method known in the art and literature may be usedto prepare the aqueous catalyst. While the specific parameters andamounts of components may vary from one method to the other, in general,one or more of the flavonoid glycosides is first solubilized in asufficient amount of water. One or more sources of metal as an aqueoussolution are combined with the stabilizer solution with vigorousagitation to form a uniform mixture. An aqueous solution containing oneor more reducing agents is then mixed with the mixture of stabilizersand metal salts with vigorous agitation to reduce the metal ions tometal. The molar ratio of metal to stabilizer may range from 1:0.1 to1:5, preferably from 1:0.2 to 1:1. The process steps and solution aretypically done at room temperature; however, temperatures may be variedto assist in solubilizing reaction components and to encourage reductionof metal ions. While not being bound by theory, the stabilizers may coator surround portions or most of the metal to stabilize the nanoparticlecolloidal catalyst solution. The metal nanoparticles may range in sizefrom 1 nm to 1000 nm or such as from 2 nm to 500 nm. Preferably thenanoparticles range in size from 2 nm to 100 nm, more preferably from 2nm to 10 nm.

One or more acids may be added to the catalyst to provide a pH range ofless than 7, preferably from 1-6.5, more preferably from 2-6. Inorganicor organic acids may be used in sufficient amounts to maintain the pH atthe desired range. Mixtures of inorganic and organic acids also may beused. Examples of inorganic acids are hydrochloric acid, sulfuric acidand nitric acid. Organic acids include mono- and polycarboxylic acids,such as dicarboxylic acids. Examples of organic acids are benzoic acidand its derivatives such as hydroxybenzoic acid, ascorbic acid,iso-ascorbic acid, malic acid, maleic acid, gallic acid, acetic acid,citric acid, oxalic acid and tartaric acid.

The nanoparticle colloidal catalysts may be used to electrolessly metalplate various substrates. Substrates include, but are not limited tomaterials including inorganic and organic substances such as glass,ceramics, porcelain, resins, paper, cloth and combinations thereof.Metal-clad and unclad materials also are substrates which may be metalplated using the catalyst. Preferably the substrates are metal-clad andunclad printed circuit boards.

Printed circuit boards include metal-clad and unclad boards withthermosetting resins, thermoplastic resins and combinations thereof,including fiber, such as fiberglass, and impregnated embodiments of theforegoing.

Thermoplastic resins include, but are not limited to, acetal resins,acrylics, such as methyl acrylate, cellulosic resins, such as ethylacetate, cellulose propionate, cellulose acetate butyrate and cellulosenitrate, polyethers, nylon, polyethylene, polystyrene, styrene blends,such as acrylonitrile styrene and copolymers and acrylonitrile-butadienestyrene copolymers, polycarbonates, polychlorotrifluoroethylene, andvinylpolymers and copolymers, such as vinyl acetate, vinyl alcohol,vinyl butyral, vinyl chloride, vinyl chloride-acetate copolymer,vinylidene chloride and vinyl formal.

Thermosetting resins include, but are not limited to, allyl phthalate,furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfuralcopolymers, alone or compounded with butadiene acrylonitrile copolymersor acrylonitrile-butadiene-styrene copolymers, polyacrylic esters,silicones, urea formaldehydes, epoxy resins, allyl resins, glycerylphthalates and polyesters.

The catalysts may be used to plate both low and high T_(g) resins. LowT_(g) resins have a T_(g) below 160° C. and high T_(g) resins have aT_(g) of 160° C. and above. Typically high T_(g) resins have a T_(g) of160° C. to 280° C. or such as from 170° C. to 240° C. High T_(g) polymerresins include, but are not limited to, polytetrafluoroethylene (PTFE)and polytetrafluoroethylene blends. Such blends include, for example,PTFE with polypheneylene oxides and cyanate esters. Other classes ofpolymer resins which include resins with a high T_(g) include, but arenot limited to, epoxy resins, such as difunctional and multifunctionalepoxy resins, bimaleimide/triazine and epoxy resins (BT epoxy),epoxy/polyphenylene oxide resins, acrylonitrile butadienestyrene,polycarbonates (PC), polyphenylene oxides (PPO), polypheneylene ethers(PPE), polyphenylene sulfides (PPS), polysulfones (PS), polyamides,polyesters such as polyethyleneterephthalate (PET) andpolybutyleneterephthalate (PBT), polyetherketones (PEEK), liquid crystalpolymers, polyurethanes, polyetherimides, epoxies and compositesthereof.

The flavonoid glycoside/precious metal nanoparticle colloidal catalystmay be used to deposit metals on the walls of through-holes or vias ofprinted circuit boards. The catalysts may be used in both horizontal andvertical processes of manufacturing printed circuit boards.

The aqueous catalysts may be used with conventional electroless metalplating baths. Typically, the metal is chosen from copper, copperalloys, nickel or nickel alloys. Preferably the metal is chosen fromcopper and copper alloys, more preferably the metal is copper.

Typically sources of copper ions include, but are not limited to watersoluble halides, nitrates, acetates, sulfates and other organic andinorganic salts of copper. Mixtures of one or more of such copper saltsmay be used to provide copper ions. Examples include copper sulfate,such as copper sulfate pentahydrate, copper chloride, copper nitrate,copper hydroxide and copper sulfamate. Conventional amounts of coppersalts may be used in the baths. Copper ion concentrations in the bathmay range from 0.5 g/L to 30 g/L or such as from 1 g/L to 20 g/L or suchas from 5 g/L to 10 g/L.

One or more alloying metals also may be included in the electrolessbaths. Such alloying metals include, but are not limited to nickel andtin. Examples of copper alloys include copper/nickel and copper/tin.Typically the copper alloy is copper/nickel.

Sources of nickel ions for nickel and nickel alloy electroless baths mayinclude one or more conventional water soluble salts of nickel. Sourcesof nickel ions include, but are not limited to nickel sulfates andnickel halides. Sources of nickel ions may be included in theelectroless alloying compositions in conventional amounts. Typicallysources of nickel ions are included in amounts of 0.5 g/L to 10 g/L orsuch as from 1 g/1 to 5 g/L.

The method steps used in metalizing a substrate may vary depending onwhether the surface to be plated is metal or dielectric. Conventionalsteps used for electrolessly metal plating a substrate may be used withthe catalysts; however, the aqueous stabilized metal catalysts do notrequire an acceleration step as in many conventional electroless platingprocesses. Accordingly, acceleration steps are preferably excluded whenusing the catalyst. In general, the catalyst is applied to the surfaceof the substrate to be electrolessly plated with a metal followed byapplication of the metal plating bath. Electroless metal platingparameters, such as temperature and time may be conventional.Conventional substrate preparation methods, such as cleaning ordegreasing the substrate surface, roughening or micro-roughening thesurface, etching or micro-etching the surface, solvent swellapplications, desmearing through-holes and various rinse andanti-tarnish treatments may be used. Such methods and formulations arewell known in the art and disclosed in the literature.

In general, when the substrate to be metal plated is a dielectricmaterial such as on the surface of a printed circuit board or on thewalls of through-holes, the boards are rinsed with water and cleaned anddegreased followed by desmearing the through-hole walls. Typicallyprepping or softening the dielectric surface or desmearing of thethrough-holes begins with application of a solvent swell.

Any conventional solvent swell may be used. The specific type may varydepending on the type of dielectric material. Examples of dielectricsare disclosed above. Minor experimentation may be done to determinewhich solvent swell is suitable for a particular dielectric material.The T_(g) of the dielectric often determines the type of solvent swellto be used. Solvent swells include, but are not limited to, glycolethers and their associated ether acetates. Conventional amounts ofglycol ethers and their associated ether acetates may be used. Examplesof commercially available solvent swells are CIRCUPOSIT CONDITIONER™3302, CIRCUPOSIT HOLE PREP™ 3303 and CIRCUPOSIT HOLE PREP™ 4120(obtainable from Rohm and Haas Electronic Materials, Marlborough,Mass.).

Optionally, the substrate and through-holes are rinsed with water. Apromoter is then applied. Conventional promoters may be used. Suchpromoters include sulfuric acid, chromic acid, alkaline permanganate orplasma etching. Typically alkaline permanganate is used as the promoter.An example of a commercially available promoter is CIRCUPOSIT PROMOTER™4130 available from Rohm and Haas Electronic Materials, Marlborough,Mass.

Optionally, the substrate and through-holes are rinsed again with water.A neutralizer is then applied to neutralize any residues left by thepromoter. Conventional neutralizers may be used. Typically theneutralizer is an aqueous alkaline solution containing one or moreamines or a solution of 3 wt % peroxide and 3 wt % sulfuric acid.Optionally, the substrate and through-holes are rinsed with water andthen dried.

After the solvent swelling and desmearing an acid or alkalineconditioner may be applied. Conventional conditioners may be used. Suchconditioners may include one or more cationic surfactants, non-ionicsurfactants, complexing agents and pH adjusters or buffers. Examples ofcommercially available acid conditioners are CIRCUPOSIT CONDITIONER™3320 and CIRCUPOSIT CONDITIONER™ 3327 available from Rohm and HaasElectronic Materials, Marlborough, Mass. Suitable alkaline conditionersinclude, but are not limited to, aqueous alkaline surfactant solutionscontaining one or more quaternary amines and polyamines. Examples ofcommercially available alkaline surfactants are CIRCUPOSIT CONDITIONER™231, 3325, 813 and 860 available from Rohm and Haas ElectronicMaterials. Optionally, the substrate and through-holes are rinsed withwater.

Conditioning may be followed by micro-etching. Conventionalmicro-etching compositions may be used. Micro-etching is designed toprovide a micro-roughened metal surface on exposed metal (e.g.innerlayers and surface etch) to enhance subsequent adhesion ofdeposited electroless and later electroplate. Micro-etches include, butare not limited to, 60 g/L to 120 g/L sodium persulfate or sodium orpotassium oxymonopersulfate and sulfuric acid (2%) mixture, or genericsulfuric acid/hydrogen peroxide. An example of a commercially availablemicro-etching composition is CIRCUPOSIT MICROETCH™ 3330 available fromRohm and Haas Electronic Materials. Optionally, the substrate is rinsedwith water.

Optionally a pre-dip is then applied to the micro-etched substrate andthrough-holes. The pre-dip helps to stabilize the catalyst bath pH andclean the metal surface. Preferably the pre-dip is used because it helpsimprove ICD reliability. Conventional pre-dip aqueous solutions ofinorganic or organic acids with a pH range typically from 3-5 may beused. An example of an inorganic acid solution is 2% to 5% hydrochloricacid. Organic acids include but are not limited to carboxylic acids,such as oxalic acid and glyoxylic acid. Optionally, the substrate isrinsed with cold water.

A stabilized flavonoid glycoside/precious metal nanoparticle colloidalcatalyst is then applied to the substrate and through-holes. Thesubstrate and through-holes optionally may be rinsed with water afterapplication of the catalyst.

The substrate and walls of the through-holes are then plated with metal,such as copper, copper alloy, nickel or nickel alloy with an electrolessbath. Preferably copper is plated on the walls of the through-holes.Plating times and temperatures may be conventional. Typically metaldeposition is done at temperatures of 20° C. to 80°, more typically from30° C. to 60° C. The substrate may be immersed in the electrolessplating bath or the electroless bath may be sprayed onto the substrate.Typically, deposition may be done for 5 seconds to 30 minutes; however,plating times may vary depending on the thickness of the metal on thesubstrate.

Optionally anti-tarnish may be applied to the metal. Conventionalanti-tarnish compositions may be used. An example of a commerciallyavailable anti-tarnish is ANTI TARNISH™ 7130 (available from Rohm andHaas Electronic Materials). The substrate may optionally be rinsed andthen the boards may be dried.

Further processing may include conventional processing by photoimagingand further metal deposition on the substrates such as electrolyticmetal deposition of, for example, copper, copper alloys, tin and tinalloys.

The catalysts may be used to electrolessly plate metals on substrates,including substrates of dielectric materials and are stable upon storageas well as during electroless metal plating since they do not readilyoxidize as compared to conventional tin/palladium catalysts. Theflavonoid glycoside stabilizers function as do stannous chloride inconventional tin/palladium catalysts except that the flavonoid glycosidestabilizers are biodegradable, thus they do not present an environmentalhazard as does stannous chloride upon disposal. The raw materials usedto make the stabilizers are readily available from plant life which isessentially ubiquitous. The flavonoid glycoside stabilized preciousmetal catalysts enable electroless metal plating without an accelerationstep, reduce or eliminate ICDs and enable good metal coverage of thesubstrate, even walls of through-holes of printed circuit boards.

The following examples are not intended to limit the scope of theinvention but are intended to further illustrate it.

Example 1

190 mg naringin dihydrate was dissolved in a beaker containing 750 ml ofDI water. The water was heated to 50° C. to assist in dissolving thenaringin dihydrate. 290 mg Na₂PdCl₄ in 25 ml of DI water was added tothe solution of naringin with stirring to form a substantially uniformorange colored solution. 93 mg NaBH₄ in 10 ml of DI water was added tothe substantially uniform solution with vigorous stir bar agitation. Thesolution changed in color from orange to black indicating the formationof colloidal palladium nanoparticles. The molar ratio of palladium metalto naringin was 1:0.3. The beaker containing the aqueous colloidalnaringin and palladium nanoparticles was then placed in a 50° C. waterbath for 12 hours to test the shelf-life stability of the nanoparticles.The pH was monitored over the 12 hour period with an ACCUMET AB15 pHmeter and fluctuated from 7 to 9 due to the formation of H₂ gas andhydroxide in solution from the excess NaBH₄. There was no observableblack precipitate at the bottom of the beaker after the 12 hour period.Accordingly, the nanoparticles were stable.

Multiple aliquots of the catalyst solution were taken from the foregoingstock solution to make a catalyst working bath with palladiumconcentration ranging from 25 ppm to 100 ppm. Either ascorbic acid orglyoxylic acid ranging from 0.1 g/L to 5 g/L was added to each aliquotto adjust the catalyst bath pH to 3-6. All samples were stable and allinitiated copper metal electroless plating on SY-1141 laminates fromShengyi.

Example 2

190 mg naringin dihydrate was dissolved in a beaker containing 750 ml ofDI water. The water was heated to 50° C. to assist in dissolving thenaringin dihydrate. 440 mg Na₂PdCl₄ in 25 ml of DI water was added tothe solution of naringin with stirring to form a substantially uniformorange colored solution. 140 mg NaBH₄ in 10 ml of DI water was added tothe substantially uniform solution with vigorous stir bar agitation. Thesolution changed in color from orange to black indicating the formationof colloidal palladium nanoparticles. The molar ratio of palladium metalto naringin was 1:0.2 and the pH of the bath was between 8 and 9. Thebeaker containing the aqueous colloidal palladium nanoparticles was thenplaced in a 50° C. water bath for 12 hours to test the stability of thenanoparticles. There was no observable formation of black precipitate atthe bottom of the beaker. The colloidal nanoparticles were stable overthe 12 hour period.

Example 3

Two sample aliquots of equal volume of the naringin/palladium colloidalnanoparticle catalyst made in Example 2 were removed from the stocksolution and diluted with DI water such that the concentration of thepalladium in each sample was 50 ppm. A sufficient amount of2,4-dihydroxybenzoic acid was added to one sample to adjust the pH to 3and the other sample had its pH adjusted to 3 by adding sufficientamounts of glyoxylic acid.

The naringin/palladium colloidal nanoparticle samples were tested forcopper electroless plating performance on two sets of six differentlaminates: TUC-662, SY-1141, SY-1000-2, IT-158, IT-180 and NPG-150.IT-158 and IT-180 were obtained from Taiwan ITEQ Corporation, NPG-150was from Nanya Corporation, TUC-662 was obtained from Taiwan UnionTechnology Corporation and SY-1141 and SY-1000 were obtained fromShengyi. The T_(g) values ranged from 140° C. to 180° C. Each laminatewas 5 cm×12 cm and had a plurality of through-holes. A surface of eachlaminate was treated as follows:

1. Each laminate was immersed into a solvent swell which includedethylene glycol dimethyl ether and water at a volume to volume ratio of1:2 for 7 minutes at 80° C.;2. Each laminate was then removed from the solvent swell and rinsed withcold tap water for 4 minutes;3. Each laminate was then treated with a permanganate aqueous solutionwhich included 1% potassium permanganate at a pH above 10 at 80° C. for10 minutes;4. Each laminate was then rinsed for 4 minutes in cold tap water;5. The laminates were then treated with a neutralizer solution of 3 wt %peroxide and 3 wt % sulfuric acid for 2 minutes at room temperature;6. Each laminate was then rinsed with cold tap water for 4 minutes;7. Each laminate was then immersed in an aqueous bath containing 3%CIRCUPOSIT CONDITIONER™ 231 aqueous acid conditioner for 5 minutes at40° C.;8. Each laminate was then rinsed with cold tap water for 4 minutes;9. MICROETCH™ 748 solution was then applied to each laminate for 2minutes at room temperature;10. Each laminate was then rinsed with cold tap water for 4 minutes;11. The laminates were then immersed in a pre-dip aqueous acid solutioncontaining 2 g/L glyoxylic acid and 1 g/L oxalic acid for 2 minutes atroom temperature;12. The laminates were then primed for 5 minutes at 40° C. with one ofthe two samples of naringin/palladium catalysts;13. The laminates were then rinsed with cold water for 4 minutes;14. The laminates were then immersed in CIRCUPOSIT™ 880 ElectrolessCopper plating bath at 40° C. and at a pH of 13 and copper was depositedon the substrates for 15 minutes;15. The copper plated laminates were then rinsed with cold water for 2minutes;16. Six of the copper plated laminates were then placed into aconventional convection oven and dried for 20 minutes at 105° C. and theother six laminates were plated with electrolytic copper for ICDperformance as described below;17. After drying, the first set of six copper plated laminates wasplaced in a conventional laboratory dessicator for 20 minutes or untilthey cooled to room temperature; and18. The first set of six copper laminates was then tested for adhesionusing the conventional Scotch tape test method.

All of the plated copper laminates had bright shinny copper deposits anda smooth appearance under a conventional microscope and passed theScotch tape test. There was no observable copper metal stuck to theScotch tape after removal of the tape from the copper laminates.

Each laminate was sectioned laterally to expose the copper plated wallsof the through-holes. Multiple lateral sections 1 mm thick were takenfrom the walls of the sectioned through-holes to determine thethrough-hole wall coverage for the boards. Ten though-holes wereexamined for each laminate. The European Backlight Grading Scale wasused. The 1 mm sections from each board were placed under a conventionaloptical microscope of 50× magnification. The quality of the copperdeposits was determined by the amount of light that was observed underthe microscope. If no light was observed the section was completelyblack and was rated a 5 on the backlight scale indicating completecopper coverage of the through-hole. If light passed through the entiresection without any dark areas, this indicated that there was verylittle to no copper metal deposition on the wall and the section wasrated 0. If sections had some dark regions as wells as light regions,they were rated between 0 and 5. The average backlight ratings for thelaminates ranged from 4.6 to 4.9. The results indicated that thecatalyst formulation was acceptable for commercial use by industrystandards.

The second set of laminates which was electrolessly copper plated alsohad a bright shinny and smooth appearance. These were electroplated inELECTROPOSIT™ 1100 Copper Electroplating bath (available from Rohm andHaas Electronic Materials LLC, Marlborough, Mass.) with a 1.5 A/dm²current density for 120 minutes to achieve an electrolytic copperthickness of 1-1.3 μm. Each laminate was then cut and the portion withthe holes to be inspected was placed in an oven at 125° C. for sixhours. The laminates subsequently underwent 6× ten second solder floatsat 288° C., followed by conventional moulding, grinding and polishing.Each was visually inspected with a conventional optical microscope forICDs. No ICDs were detected on the six laminates.

Example 4

The plating method described in Example 3 above was repeated except thatthe concentration of the palladium in the colloidal nanoparticlecatalyst was increased to 100 ppm. The samples appeared stable with noobservable precipitation. All of the plated copper laminates had brightshinny copper deposits with good morphology and passed the Scotch tapetest. There was no observable copper metal stuck to the Scotch tapeafter removal of the tape from the copper laminates. The averagebacklight test results ranged from 4.6 to 4.9. No ICDs were observed onany of the laminates.

Example 5

1.2 g rutin trihydrate was added to a beaker containing 800 ml DI water.1 N NaOH solution with stirring was added to adjust the solution pH to11.2. In a separate beaker, 1.65 g silver p-toluenesulfonate wasdissolved in 40 ml DI water. The silver ion solution was added to therutin trihydrate solution with vigorous stirring. A minute after mixingthe two solutions, a few milliliters of the solution were taken out anddiluted for UV-vis measurement. The spectra were measured using a UV-Visspectrophotometer 8453 from Agilent. The UV-Vis spectra showed a strongabsorption peak around 412 nm, indicating the formation of silvernanoparticles. The molar ratio of silver metal to rutin trihydrate was1:0.3. The beaker containing the aqueous catalyst solution was placed ina 50° C. water bath for 12 hours to test its stability. No precipitatewas observed. The silver nanoparticle solution was tested as a catalystfor electroless copper plating by taking ten aliquots of equal volumefrom the stock solution and then each aliquot was diluted with DI waterto 320 ppm silver and adjusted to pH 3-6 with ascorbic acid ordihydroxybenzoic acid. Ten laminates with a plurality of through-holeswere provided: SY-1141, SY-1000-2, TUC-752, 370HR (from Isola), andNP-175 (from Nanya). The catalyst initiated electroless plating on allof the laminates. All of the laminates were bright shinny and smooth inappearance. Five of the laminates were tested for adhesion. All passedthe Scotch tape test. Backlight performance was tested on the fivelaminates and the average values were between 4.5 and 4.8, which aregenerally accepted by industrial standards. The remaining five laminateswere analyzed for ICDs as described in Example 3. No ICDs were detectedon the five laminates.

Example 6

Robinin/silver colloidal nanoparticle catalyst is prepared by dissolving1.07 g robinin in a beaker containing 800 ml DI water at roomtemperature. 1 N NaOH solution is added with stirring to adjust thesolution pH to 11. In a separate beaker, 1.42 g silverp-toluenesulfonate is dissolved in 40 ml DI water. The silver ionsolution is added to the robinin solution with vigorous stirring. Themolar ratio of silver metal to robinin is 1:0.28. The beaker containingthe aqueous catalyst solution is placed in a 50° C. water bath for 12hours to test its stability. After 12 hours the solution is observed andno precipitate is expected indicating that the catalyst is still stable.

The rubinin/silver catalyst is diluted with sufficient amount of DIwater such that the concentration of the catalyst is 300 ppm silver. ThepH of the catalyst solution is adjusted to 3 with a sufficient amount ofascorbic acid. The catalyst solution is used to electrolessly platecopper on TUC-662 laminates with a plurality of through-holes accordingto the preparation and plating procedure described in Example 3. Theplated copper laminates are expected to be bright and shinny with asmooth appearance and pass the Scotch tape test. No ICDs are expected tobe observed on the laminates. The copper plated laminates are sectionedand 10 through-holes of each laminate are examined for backlightperformance. The average backlight values are expected to range from 4.5to 4.8.

Example 7

Hesperidin/silver colloidal nanoparticle catalyst is prepared bydissolving 1.65 g hesperidin in a beaker containing 800 ml DI water atroom temperature. 1 N NaOH solution with stirring is added to adjust thesolution pH to 11. In a separate beaker, 2.2 g silver p-toluenesulfonateis dissolved in 60 ml DI water. The silver ion solution is added to thehesperidin solution with strong stirring. The molar ratio of silver tohesperidin is 1:0.34. The beaker containing the aqueous catalystsolution is placed in a 50° C. water bath for 12 hours to test itsstability. After 12 hours the solution is observed and no precipitate isexpected indicating that the catalyst is still stable.

The hesperidin/silver catalyst is diluted with sufficient amount of DIwater such that the concentration of the catalyst is 300 ppm silver. ThepH of the catalyst solution is adjusted to 3 with a sufficient amount ofdihydroxybenzoic acid. The catalyst solution is used to electrolesslyplate copper on SY-1141 laminates with a plurality of through-holesaccording to the preparation and plating procedure described in Example3. The plated copper laminates are expected to be bright and shinny witha smooth surface and pass the Scotch tape test. No ICDs are expected tobe observed on the laminates. The copper plated laminates are sectionedand 10 through-holes of each laminate are examined for backlightperformance. The average backlight values are expected to range from 4.5to 4.8.

Example 8

Diosmin/palladium colloidal nanoparticle catalyst is prepared bydissolving 195 mg diosmin in a beaker containing 500 ml DI water at roomtemperature. With stirring, 300 mg Na₂PdCl₄ in 30 ml DI water is addedand the mixture is vigorously stirred using air agitation. 100 mg NaBH₄in 10 ml DI water is then added to the solution with vigorous airagitation. The solution becomes black indicating reduction of palladiumions to palladium metal and the formation of palladium nanoparticles.The molar ratio of palladium to diosmin is 1:0.3. The solution of thecatalyst has a pH from 8 to 9 as measured using an ACCUMET AB 15 pHmeter. The beaker containing the aqueous catalyst solution is placed ina 50° C. water bath for 12 hours to test its stability. After 12 hoursthe solution is observed and no observable precipitate is expectedindicating that the catalyst is still stable.

2 sample aliquots of the diosmin/palladium catalyst of equal volume areremoved from the above stock solution and diluted with sufficient amountof DI water such that the concentrations of the catalyst are 50 ppmpalladium. The pH of the aliquots is adjusted to 3.5 with a sufficientamount of ascorbic acid. Each is used to electrolessly plate copper onSY-1141 laminates with a plurality of through-holes according to thepreparation and plating procedure described in Example 3. The platedcopper laminates are expected to be bright with a smooth surface andpass the Scotch tape test. The two copper plated laminates are sectionedand 10 through-holes are examined for backlight performance. The averagebacklight values are expected to range from 4.5 to 5.

Example 9

Quercitrin/palladium colloidal nanoparticle catalyst is prepared bydissolving 195 mg qiercitrin in a beaker containing 750 ml DI water atroom temperature. With stirring, 300 mg Na₂PdCl₄ in 30 ml DI water isadded and the mixture is vigorously stirred using air agitation. 100 mgNaBH₄ in 15 ml DI water is then added to the solution with vigorous airagitation. The solution becomes black indicating reduction of palladiumions to palladium metal and the formation of palladium nanoparticles.The solution of the catalyst has a pH of 7 to 8 as measured using anACCUMET AB 15 pH meter. The molar ratio of palladium to quecitrin is1:0.4. The beaker containing the aqueous catalyst solution is placed ina 50° C. water bath for 12 hours to test its stability. After 12 hoursthe solution is observed and no observable precipitate is expectedindicating that the catalyst is still stable.

2 sample aliquots of the quercitrin/palladium catalyst of equal volumeare removed from the above stock solution and diluted with sufficientamount of DI water such that the concentrations of the catalyst are 50ppm palladium. The pH of the aliquots is adjusted to 4 with a sufficientamount of ascorbic acid. Each is used to electrolessly plate copper onNPG-150 laminates according to the method described in Example 3. Theplated copper laminates are expected to be bright with a smooth surfaceand are expected to pass the Scotch tape test. The two copper platedlaminates are sectioned and 10 through-holes are examined for backlightperformance. The average backlight values are expected to range from 4.5to 5.

Example 10

Myricetin 3-rhamnoside/silver colloidal nanoparticle catalyst isprepared by dissolving 1.04 g myricetin 3-rhamnoside in a beakercontaining 750 ml DI water at room temperature. With stirring, 1 N NaOHsolution is added to adjust the solution pH to 11. In a separate beaker,1.4 g silver p-toluenesulfonate is dissolved in 40 mL DI water. Thesilver ion solution is added to the myricetin 3-rhamnoside solution withvigorous stirring. The molar ratio of silver to myricetin 3-rhamnosideis 1:0.4. The beaker containing the aqueous catalyst solution is placedin a 50° C. water bath for 12 hours to test its stability. After 12hours the solution is observed and no precipitate is expected indicatingthat the catalyst is still stable.

The myricetin 3-rhamnoside/silver catalyst is diluted with sufficientamount of DI water such that the concentration of the catalyst is 300ppm silver. The pH of the catalyst solution is adjusted to 3 with asufficient amount of dihydroxybenzoic acid. The catalyst solution isused to electrolessly plate copper on NPG-150 laminates withthrough-holes according to the preparation and plating proceduredescribed in Example 3. The plated copper laminates are expected to bebright and shinny with smooth surfaces and pass the Scotch tape test. NoICDs are expected to be observed on the laminates. The copper platedlaminates are sectioned and 10 through-holes of each laminate areexamined for backlight performance. The average backlight values areexpected to range from 4.5 to 4.8.

What is claimed is:
 1. An aqueous catalyst solution comprising one ormore reducing agents and nanoparticles comprising one or more preciousmetals and one or more flavonoid glycosdies and hydrates thereof.
 2. Theaqueous catalyst solution of claim 1, wherein the one or more flavanoidglycosides have a general formula:

wherein R₁, R₂, R₃ and R₄ are independently hydrogen, hydroxyl, linearor branched (C₁-C₆)alkoxy, linear or branched (C₁-C₆)acetate or—O-carbohydrate; Z₁ is >C═O or a structure having formula:

wherein R₅ and R₆ are independently hydrogen, or ring C, when Z₁ forms adouble bond with the carbon at position 3 of ring B, only one of R₅ andR₆ is present and joined to the carbon at position 2; Z₂ is a structurehaving formula:

wherein R₇ and R₈ are independently hydrogen, hydroxyl, ring C or—O-carbohydrate, when Z₂ forms a double bond with the carbon at position2 or 4 of ring B, only one of R₇and R₈ is present and joined with thecarbon of position 3; Z₃ is >C═O or a structure having formula:

wherein R₉ and R₁₀ are hydrogen, when Z₃ forms a double bond with thecarbon at position 3 of ring C, only one of R₉ and R₁₀ is present andjoined to the carbon at position 4; and

R_(1′), R_(2′), R_(3′), R_(4′) and R_(5′) of ring C are independentlyhydrogen, hydroxyl, linear or branched (C₁-C₆)alkoxy, linear or branched(C₁-C₆)acetate or —O-carbohydrate, wherein at least one of ring A, B orC includes —O-carbohydrate.
 3. The aqueous catalyst solution of claim 1,wherein the nanoparticles are 1 nm to 1000 nm.
 4. The method of claim 1,wherein a molar ratio of the one or more precious metals to the one ormore flavonoid glycosides and hydrates ranges from 1:0.1 to 1:5.
 5. Amethod comprising: a. providing a substrate; b. applying an aqueouscatalyst solution to the substrate, the aqueous catalyst solutioncomprises one or more reducing agents and nanoparticles comprising oneor more precious metals and one or more flavonoid glycosides andhydrates; and c. electrolessly depositing metal onto the substrate usingan electroless metal plating bath.
 6. The method of claim 5, wherein thesubstrate comprises a plurality of through-holes.
 7. The method of claim5, wherein the electroless metal plating bath is chosen from a copper,copper alloy, nickel and nickel alloy bath.